The AR600 that I have is only half inch, but it's so hard that beyond 200 yards, nothing I've shot at it even leaves a mark. I haven't and won't shoot it with a 300 RUM, It may leave a mark and I don't want that. I've shot regular 3/4 steel with a RUM and it really leave a crater. I need mine very clean so I can see the hits out to 900 yards. The hardest hitting gun I've shot it with is my 308 and .270. I also don't shoot it with full Metal jackets. That said, target and hunting bullets explode on it. With fresh paint, you can' tell it's every been shot. I don't think the 300RUM will dent it at 500, but I'll bet you it will at 100 or 200.
AR 550 and 600 are pretty expensive and hard to find. I got mine from EBAY. But most of the steel targets on there are mild steel, your RUM will destroy them. Look for the AR 600 rating, you can tell by the PRICE.